Electrochemical sensor for the detection of hydrogen chloride

ABSTRACT

The present invention provides an electrochemical sensor for the detection of hydrogen chloride. In general, the electrochemical sensor comprises a housing having disposed therein a working electrode, a reference electrode and a counter electrode. The electrochemically active surface of the working electrode preferably comprises a gold film having a thickness of approximately 1000 to 3000 Å. Electrical connection is maintained between the working electrode and the counter electrode via an electrolyte present within the housing. The electrochemical gas sensor preferably further comprises circuitry for maintaining the working electrode at a potential in the range of approximately 1025 to approximately 1400 mV versus the normal hydrogen electrode. The present invention also provides a method of using such a sensor to detect hydrogen chloride.

RELATED APPLICATIONS

This is a divisional of U.S. Ser. No. 08/887,068 filed on Jul. 2, 1997now U.S. Pat. No. 5,908,546.

FIELD OF THE INVENTION

The present invention relates to an electrochemical sensor, andparticularly, to an electrochemical sensor for detecting hydrogenchloride.

BACKGROUND OF THE INVENTION

In an electrochemical gas sensor, the gas to be measured typicallypasses from the atmosphere into the sensor housing through a gas porousor gas permeable membrane to a working electrode (sometimes called asensing electrode) where a chemical reaction occurs. A complementarychemical reaction occurs at a second electrode known as a counterelectrode (or an auxiliary electrode). The electrochemical sensorproduces an analytical signal via the generation of a current arisingdirectly from the oxidation or reduction of the analyte gas (that is,the gas to be detected) at the working and counter electrodes. Acomprehensive discussion of electrochemical gas sensors is provided inCao, Z. and Stetter, J. R., "The Properties and Applications ofAmperometric Gas Sensors," Electroanalysis, 4(3), 253 (1992), thedisclosure of which is incorporated herein by reference.

To be useful as an electrochemical sensor, a working and counterelectrode combination must be capable of producing an electrical signalthat is (1) related to the concentration of the analyte and (2)sufficiently strong to provide a signal-to-noise ratio suitable todistinguish between concentration levels of the analyte over the entirerange of interest. In other words, the current flow between the workingelectrode and the counter electrode must be measurably proportional tothe concentration of the analyte gas over the concentration range ofinterest.

In addition to a working electrode and a counter electrode, anelectrochemical sensor often includes a third electrode, commonlyreferred to as a reference electrode. A reference electrode is used tomaintain the working electrode at a known voltage or potential. Thereference electrode should be physically and chemically stable in theelectrolyte and carry the lowest possible current to maintain a constantpotential.

Electrical connection between the working electrode and the counterelectrode is maintained through an electrolyte. The primary functions ofthe electrolyte are: (1) to efficiently carry the ionic current; (2) tosolubilize the analyte gas; (3) to support both the counter and theworking electrode reactions; and (4) to form a stable referencepotential with the reference electrode. The primary criteria for anelectrolyte include the following: (1) electrochemical inertness; (2)ionic conductivity; (3) chemical inertness; (4) temperature stability;(5) low cost; (6) low oxicity; (7) low flammability; and (8) appropriateiscosity.

In general, the electrodes of an electrochemical cell provide a surfaceat which an oxidation or a reduction reaction occurs to provide amechanism whereby the ionic conduction of the electrolyte solution iscoupled with the electron conduction of the electrode to provide acomplete circuit for a current.

The measurable current arising from the cell reactions because of theanalyte in the electrochemical cell is directly proportional to the rateof reaction. Preferably, therefore, a high reaction rate is maintainedin the electrochemical cell. For this reason, the counter electrodeand/or the working electrode of the electrochemical cell generallycomprise an appropriate electrocatalyst on the surface thereof toenhance the reaction rate. If the reaction rate of either half cellreaction is impeded, resulting in a low exchange current density, theequilibrium current of the electrochemical cell may be easily perturbedduring measurement. Such deviation can result in undesirable sidereactions and/or nonlinear behavior over the range of analyteconcentrations desired to be detected.

The type, rate, and efficiency of the chemical reactions within anelectrochemical gas sensor are controlled, in significant part, by thematerial(s) used to make the working electrode and counter electrode.Indeed, extensive research efforts are expended to develop improvedworking electrodes, counter electrodes and electrochemical systemsgenerally. See Cao, supra.

In the case of electrochemical sensors for the detection of hydrogenchloride (HCl), these efforts have met with somewhat limited success. Inthat regard, currently available electrochemical sensors for thedetection of HCl suffer from a number of significant drawbacks,including: (1) poor response time; (2) short service time; (3)sensitivity to changes in temperature; (4) sensitivity to changes inhumidity; and (5) susceptibility to interference from orcross-sensitivity to gases other than HCl (for example, hydrogen sulfide(H₂ S)).

It is desirable, therefore, to develop new electrochemical sensors andelectrodes for use in such electrochemical sensors for the detection ofhydrogen chloride which mitigate or substantially eliminate one or moreof the above drawbacks.

SUMMARY OF THE INVENTION

The present invention provides an electrochemical sensor for thedetection of hydrogen chloride. In general, the electrochemical sensorpreferably comprises a housing having disposed therein a workingelectrode, a reference electrode and a counter electrode. Theelectrochemically active surface of the working electrode preferablycomprises a gold film having a thickness of approximately 1000 to 3000Å. Electrical connection is maintained between the working electrode andthe counter electrode via an electrolyte present within the housing. Theelectrochemical gas sensor preferably further comprises circuitry formaintaining the working electrode at a potential in the range ofapproximately 1025 mV to approximately 1400 mV versus the normalhydrogen electrode (that is, the operating potential is preferable inthe range of approximately +1050 mV to approximately +1400 mV relativeto the normal hydrogen electrode). More preferably, the workingelectrode is maintained at a potential in the range of approximately1050 mV to approximately 1350 mV versus the normal hydrogen electrode.Most preferably, the working electrode is maintained as a potential inthe range of approximately 1100 mV to approximately 1200 mV versus thenormal hydrogen electrode.

Preferably, the working electrode comprises a film of gold sputtercoated over a porous, water-resistant membrane, such as a GoreTex®membrane. The electrochemically active surface of the counter electrodepreferably comprises platinum. Similarly, the electrochemically activesurface of the reference electrode preferably comprises platinum.

The present invention also provides a method of using an electrochemicalgas sensor comprising a working electrode having an electrochemicallyactive surface comprising a gold film of a thickness of approximately1000 Å to 3000 Å for the detection of hydrogen chloride. The methodpreferably comprises the steps of:

a. placing the electrochemical gas sensor in communicative connectionwith an environment containing hydrogen chloride such that hydrogenchloride can react at the working electrode;

b. measuring the current flow between the working electrode and thecounter electrode to obtain a measurement of the concentration ofhydrogen chloride in the environment.

The method preferably further comprises the step of:

c. maintaining the working electrode at a potential in the range ofapproximately 1025 to approximately 1400 mV versus the normal hydrogenelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic representation of a cross-sectionalview of an embodiment of an electrochemical gas sensor of the presentinvention.

FIG. 2 is a perspective view of an embodiment of the present counterelectrode.

FIG. 3 is a perspective view of an embodiment of the present referenceelectrode.

FIG. 4 is a graph of an interferent study comparing the output of asensor of the present invention with the output of a hydrogen chloridesensor having a working electrode fabricated with gold powder in thepresence of hydrogen chloride and the interferent hydrogen sulfide.

FIG. 5A plots a study of the output of an electrochemical sensor of thepresent invention as a function of cycling operating potential in thepresence of the interferent hydrogen sulfide.

FIG. 5B plots a study of the output of an electrochemical sensor havingan electrode fabricated from relatively low surface area powdered goldas a function of cycling operating potential in the presence of theinterferent hydrogen sulfide.

FIG. 6A plots a study of the output of an electrochemical sensor of thepresent invention as a function of cycling operating potential in thepresence of the analyte hydrogen chloride.

FIG. 6B plots a study of the output of an electrochemical sensor havingan electrode fabricated from relatively low surface area powdered goldas a function of cycling operating potential in the presence of theinterferent hydrogen chloride.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in FIG. 1, electrochemical hydrogen chloride sensor 1preferably comprises a housing 5, enclosing a working electrode 10, areference electrode 20 and a counter electrode 30. In fabricatingelectrochemical hydrogen chloride sensors 1 for use in the presentstudies a porous spacer or wick 35 was first placed within housing 5.Counter electrode 30 was then placed into housing 5. A porous spacer orwick 40 was preferably then placed within housing 5 followed byreference electrode 20. A porous wick 50 was subsequently placed withinhousing 5 followed by working electrode 10.

After placement of working electrode 10 within housing 5, the perimeterof working electrode 10 was heat sealed to housing 5. The interior ofhousing 5 was then filled with an electrolyte such as sulfuric acid (H₂SO₄) via opening 70. Upon filling of the interior of housing 5 withelectrolyte, opening 70 was sealed, preferably via heat sealing using awater resistant membrane such as a GoreTex® film (not shown). A detaileddiscussion of a preferred assembly, including connection of electricalleads, for electrochemical gas sensor 1 is set forth in U.S. Pat. No.5,338,429, the disclosure of which is incorporated herein by reference.

Wicks 40 and 50 operate to prevent physical contact of the electrodesbut allow the liquid electrolyte to contact the electrodes and therebyprovide ionic connection between working electrode 10 and counterelectrode 30. Preferably, the electrolyte used in electrochemicalhydrogen chloride sensor 1 is sulfuric acid.

The electrochemically active surface of working electrode 10 preferablycomprises gold (Au). Working electrodes 10 for use in electrochemicalsensors 1 for the present studies were preferably fabricated viasputtering deposition of gold upon a gas porous, water resistantmembrane such as GoreTex film as known in the art. Thin films of goldcan also be deposited upon a GoreTex film using other methods such asevaporative vapor deposition. As known in the art, GoreTex films providea very good support for an electrochemically active material and alsoprovide a good gas porous diffusion barrier, allowing the analyte gas todiffuse into the electrochemical sensor while preventing the escape ofliquid electrolyte. The gold layer is preferably deposited as arelatively thin film so that permeability is maintained, but not so thinas to result in isolated islands of gold and associated high resistance.In that regard, the gold is preferably sputtered to a thickness ofapproximately 1000 Å to 3000 Å. More preferably, the gold is sputteredto a thickness of approximately 1900 to 2800 Å. Such thin films alsoprovide improved response times.

Counter electrodes 30 and reference electrodes 20 for use inelectrochemical sensors 1 for the present studies may be fabricated viasilk screen deposition of an ink comprising a suitable electrochemicallyactive material. In general, the electrochemically active material usedin counter electrodes 30 and reference electrodes 20 is not critical tothe operation of electrochemical sensor 1. The ink may deposited viasilk screening upon a GoreTex film as known in the art. The ink may alsobe deposited using hand painting techniques as known in the art.Preferably, a film of electrochemically active material having athickness in the range of approximately 1 to 10 mil is deposited. Thesupport for the film can comprise other electrically conductivematerials such as, for example, electrically conductive carbon.

The electrochemically active surfaces of counter electrode 30 andreference electrode 20 preferably comprise platinum (Pt). In the case ofplatinum, counter electrodes 30 and reference electrodes 20 for thepresent studies were preferably fabricated via hand painting depositionof an ink comprising platinum powder upon a GoreTex film.

After deposition of the films upon counter electrode 30 and referenceelectrode 20 as described above, the films were preferably sintered tofix the electrochemically active material upon the substrate GoreTexsuch as is described in U.S. Pat. No. 4,790,925, the disclosure of whichis incorporated herein by reference.

As illustrated in FIGS. 1 and 2, counter electrode 30 is preferablyshaped in the general form of an annulus or ring. As illustrated inFIGS. 1 and 3, reference electrode 20 is preferably shaped in agenerally circular form (that is, in the general shape of a disk). Asclear to those skilled in the art, however, counter electrode 30,reference electrode 20 and working electrode 10 of electrochemicalsensor 1 can be fabricated in many different shapes.

Preferably, electrochemical hydrogen chloride sensor 1 is subjected to a"cook-down" or "equilibration" period before use thereof to provide anadequately stable and low baseline. During the cook-down orequilibration period, electrochemical sensor 1 is stored at ambientconditions for a defined period of time. As common in the art,electrochemical sensor 1 is preferably maintained at operating potentialduring the cook-down period. The operating potential of theelectrochemical sensor 1 is preferably +1025 mV to approximately+1400 mVversus the normal hydrogen electrode (or approximately +25 mV toapproximately +400 mV versus the platinum/air reference electrode). Mostpreferably, the operating potential of the electrochemical sensor 1 ispreferably +1100 mV to +1200 mV versus the normal hydrogen electrode.For the present studies, working electrode 10 was preferably maintainedat a potential of 1200 mV versus the normal hydrogen electrode duringthe cook-down period.

Preferably, a substantially stable baseline in the range ofapproximately -0.25 μA to approximately +0.25 μA is achieved during thecook-down period. It has been found that a cook-down period ofapproximately one hour is sufficient to provide an adequate baseline forelectrochemical hydrogen chloride sensor 1. Briefer cook-down periodshave not yet been investigated, however. Electrochemical hydrogenchloride sensors 1 used in the studies discussed below were subjected toa one hour cook-down period.

Response time is an empirical measure of the speed of response of asensor and is critically dependent on the manner in which the test isperformed (for example, the length of time the experiment lasts and/orthe time at which the sensor reaches 100% of its final output). In thepresent studies, the response time was based upon a ten (10) minuteexposure to test gas. Response time was generally tabulated as the 90%response time (t₉₀) unless otherwise indicated. The t₉₀ response time isthe time, in seconds, required for the sensor to reach 90% of theresponse or output obtained after ten minutes of exposure to test gas.The sensitivity (in units of μA/ppm HCl) was established as the sensoroutput after ten (10) minutes of exposure to hydrogen chloride.

The present studies included linear and cyclic voltammetries performedusing an EG&G model 263A potentiostat controlled by Model 270/250Research Electrochemistry software also produced by EG&G. This softwarecontrolled both the applied potential and the data acquisition in cyclicstudies. Further studies at single potentials were done using a BASVoltammograph to apply a fixed potential. Data was recorded with a stripchart recorder or a Fluke digital multimeter. Sensor response in μA andresponse time (reported as t₉₀) were determined from the strip charts asknown in the art.

All the sensor cells in the studies had a single 0.230 inch diameterinlet hole to allow the test gas to enter the sensor cells. An averageoutput of approximately 0.30 μA/ppm was obtained under theseexperimental conditions. As clear to one of ordinary skill in the art,sensitivity can generally be increased by increasing the total surfacearea of such inlet holes to allow more gas to enter the sensor cell. Thesignal/noise ratio of electrochemical sensors of the present inventionwas found to be larger than provided by previous electrochemical sensorsfor the detection of hydrogen chloride. The electrochemical sensors ofthe present invention were found to provide a signal/noise ratiosuitable to measure concentrations of hydrogen chloride at least as lowas 0.1 ppm. Prior electrochemical sensors for the detection of hydrogenchloride typically cannot resolve concentrations lower than 0.5 ppm.

The electrochemical sensors of the present invention were found toprovide a substantially linear signal over at least the range ofapproximately 0 to 50 ppm hydrogen chloride. Concentration higher than50 ppm were not studied, however. This response time of the presentsensors was found to be less than approximately 50 seconds to 90%. Theresponse time is approximately twice as fast as possible with previouselectrochemical sensors for the detection of hydrogen chloride. The t₉₀was found to be substantially constant over the life of the sensor.

As typical of sensors comprising aqueous electrolytes, the sensitivityof the sensors of the present invention was found to be affected byhumidity. Sensitivity was found to decrease if the sensor was stored inlow humidity, whereas sensitivity was found to increase if the sensorwas stored in a humid environment. In general, sensitivity was found todecrease if the sensors were stored in an environment having a relativehumidity of less than approximately 15%. Preferably, therefore, thesensors of the present invention are stored in an environment having arelative humidity in the range of approximately 15 to 90%. It isbelieved that the drop in sensor sensitivity at low humidity is a resultof loss of solution contact. This "drying" and the resultant sensitivityloss at low humidity are reversible upon exposure of the sensor toambient conditions in which the relative humidity is preferably in therange of approximately 15 to 90%.

Unlike currently available sensors for the detection of hydrogenchloride, however, the sensors of the present invention were found to berelatively insensitive to short term changes in humidity. In thatregard, previous hydrogen chloride sensors exhibit an immediate changein sensor output as a result of changes in humidity.

Similarly, the sensors of the present invention were found to berelatively insensitive to short term changes in temperature, whereasprevious hydrogen chloride sensors exhibit an immediate change in sensoroutput as a result of changes in temperature. Table 1 sets forth acomparison of the output of an electrochemical sensor of the presentinvention and the output of a CiTicel 7HL hydrogen chloride sensoravailable from City Technology of Portsmouth, England at varioustemperature. As illustrated by the data of Table 1, the output of theelectrochemical sensor of the present invention is less sensitive tochanges in temperature than is the output of the City Technology sensor.The sample gas in the studies of Table 1 was 40 ppm HCl in nitrogen.

                  TABLE 1                                                         ______________________________________                                                 Present        CiTicel 7HL                                                    Electrochemical Sensor                                                                       Electrochemical Sensor                                Temperature                                                                            (ppm)          (ppm)                                                 ______________________________________                                        25° C.                                                                 Zero     0              0                                                     Span     40             40                                                    40° C.                                                                 Zero     0              4                                                     Span     39             42                                                    0° C.                                                                  Zero     0              -3                                                    Span     39             31                                                    ______________________________________                                    

The results of several interferent studies are set forth in Table 2. Thedata provided for each interferent gas correspond to the sensor output(that is, the indicated concentration of hydrogen chloride in ppm) uponexposure of the sensor to the indicated amount (set forth in ppm) of theinterferent gas. The results indicate that the present sensor is lesssusceptible to erroneous results arising from the presence of theinterferent gases studied than previous hydrogen chloride sensors.

                  TABLE 2                                                         ______________________________________                                        Sample  40 ppm HCl                                                                              2 ppm Cl.sub.2                                                                          10 ppm H.sub.2 S                                                                      10 ppm SO.sub.2                           ______________________________________                                        Sensor  40        0         15      0                                         Response                                                                      (ppm HCl                                                                      indicated)                                                                    ______________________________________                                    

FIG. 4 illustrates the output of an electrochemical sensor of thepresent invention (line A) as compared to the output of anelectrochemical sensor comprising a working electrode fabricated fromrelatively "low" specific surface area powdered gold (0.18 to 0.34 m²/g) (line B) in the presence of hydrogen chloride and hydrogen sulfideat an operating potential of +1200 mV versus the normal hydrogenelectrode.

As represented by line A, an electrochemical sensor having a workingelectrode comprising a sputter coated gold film was exposed to a samplegas. At time A1 a concentration of approximately 42 ppm hydrogenchloride was introduced into the sample gas. The hydrogen chloridesupply was discontinued at time A2. A concentration of approximately 42ppm hydrogen chloride was once again introduced into the sample gas attime A3 and was discontinued at time A4. The somewhat slow response timeobserved after time A1 is believed to be a result of lag time in thesupply of hydrogen chloride as the hydrogen chloride cylinder was turnedoff between experiments to conserve gas. At time A5, a concentration ofapproximately 42 ppm hydrogen sulfide was introduced into the samplegas.

As represented by line B, an electrochemical sensor having a workingelectrode comprising low specific surface area powdered gold (0.18 to0.34 m² /g) was exposed to a sample gas. At time B1 a concentration ofapproximately 42 ppm hydrogen chloride was introduced into the samplegas. The hydrogen chloride supply was discontinued at time B2. At timeB3, a concentration of approximately 42 ppm hydrogen sulfide wasintroduced into the sample gas. At time B3, the scale of the graph ofFIG. 4 changes from a full scale of approximately 0.05 mA to a fullscale of approximately 0.1 mA.

FIGS. 5A illustrates the output of an electrochemical sensor of thepresent invention as a function of cycling operating potential in thepresence of hydrogen sulfide. For comparison, FIG. 5B illustrates theoutput of an electrochemical sensor comprising a working electrodefabricated from relatively "low" specific surface area powdered gold(0.18 to 0.34 m² /g) as a function of cycling operating potential in thepresence of hydrogen sulfide. In the studies of FIG. 5A and 5B, theoperating potential of the sensor was cycled from -1100 to +1400 mV(versus the NHE) and back to -1100 mV at a rate of 0.5 mV per second.The cycle was repeated three time continuously with approximately 40 ppmof hydrogen sulfide gas applied. FIGS. 6A and 6B, illustrate similarstudies with a sputtered gold working electrode and a low specificsurface area powdered gold (0.18 to 0.34 m² /g) working electrode,respectively, in the presence of hydrogen chloride.

The present inventors have found that, unlike typical workingelectrodes, a working electrode comprising a relatively thin film ofgold (see FIG. 4 and FIG. 5A) relatively quickly loses much of itssensitivity to hydrogen sulfide. Indeed, comparing FIGS. 5A and 5B, itis seen that the response to hydrogen sulfide is substantially reducedafter the first cycle in the case of a working electrode comprising asputtered film of gold, while the response to hydrogen sulfide of aworking electrode comprising low surface area gold is unaffected byprevious exposure to hydrogen sulfide at the levels of the study overthe course of the study. It is believed that application of hydrogensulfide to such an electrode at the operating potentials of the presentinvention poisons the electrode against further detection of hydrogensulfide. The sensitivity of the working electrode to hydrogen chlorideremains relatively unchanged, however. This result is not observed withworking electrodes fabricated from powdered gold using typicalfabrication techniques.

After poisoning as described above, the cross sensitivity of the sensorsof the present invention to the presence of hydrogen sulfide isgenerally found to be less than or equal to approximately 1:1 (that is,1 ppm of hydrogen chloride is indicated for each 1 ppm of hydrogensulfide present in the test sample). The data set forth in Table 2 isfor a sensor that has not undergone such a hydrogen sulfide poisoningprocedure. The cross sensitivity of previous hydrogen chloride sensorsto hydrogen sulfide is generally found to be approximately 2:1 (that is,2 ppm of hydrogen chloride is indicated for each 1 ppm of hydrogensulfide present in the test sample).

The surface area of a working electrode fabricated to have a thin filmof gold thereon is in the range of approximately two to three times theactual area of the electrode. The surface of a working electrodefabricated using standard silk screen or hand painting techniques witheven relatively low-specific-surface-area powdered gold is in the rangeof approximately 50 to 100 times the actual area of the electrode. It isbelieved that in the case of a working electrode comprising a relativelythin film of gold the sulfur of hydrogen sulfide bonds to the gold andprevents further reaction of hydrogen sulfide. This bonding, however,does not substantially affect the reaction of hydrogen chloride at theworking electrode. This effect is not observed in the case of highsurface area electrodes.

Although the present invention has been described in detail inconnection with the above examples, it is to be understood that suchdetail is solely for that purpose and that variations can be made bythose skilled in the art without departing from the spirit of theinvention except as it may be limited by the following claims.

What is claimed is:
 1. An electrochemical sensor for the detection ofhydrogen chloride, comprising: a housing, the housing having disposedtherein a working electrode, a reference electrode and a counterelectrode, the electrochemically active surface of the working electrodecomprising a gold film having a thickness in the range of approximately1000 Å to approximately 3000 Å, the gold film of the working electrodehaving been exposed to an atmosphere containing hydrogen sulfide of asufficient concentration for a sufficient length of time to decrease asensitivity of the working electrode to hydrogen sulfide by at leastapproximately 50 percent, the electrochemical sensor further comprisingan electrolyte present within the housing maintaining electricalconnection between the working electrode and the counter electrode; andcircuitry maintaining the working electrode at a potential in the rangeof approximately 1025 mV to approximately 1400 mV versus the normalhydrogen electrode.
 2. The electrochemical sensor of claim 1 wherein thecircuitry maintains the working electrode at a potential in the range ofapproximately 1050 mV to approximately 1350 mV versus the normalhydrogen electrode.
 3. The electrochemical sensor of claim 1 wherein thecircuitry maintains the working electrode at a potential in the range ofapproximately 1100 mV to approximately 1200 mV versus the normalhydrogen electrode.
 4. The electrochemical sensor of claim 1 wherein thethickness of the gold film is in the range of approximately 1900 Å toapproximately 2800 Å.
 5. The electrochemical sensor of claim 1 whereinthe gold film is sputter coated over a porous membrane.
 6. Theelectrochemical sensor of claim 1 wherein the electrochemically activesurface of the counter electrode comprises platinum.
 7. Theelectrochemical sensor of claim 1 wherein the electrochemically activesurface of the reference electrode comprises platinum.
 8. Theelectrochemical sensor of claim 1 wherein the electrochemically activesurfaces of both the reference electrode and the counter electrodecomprise platinum.
 9. The electrochemical sensor of claim 1 wherein theelectrolyte comprises sulfuric acid.
 10. The electrochemical sensor ofclaim 1 wherein the sensitivity of the working electrode to hydrogensulfide has been decreased by at least approximately 50 percent.
 11. Anelectrochemical sensor for the detection of hydrogen chloride,comprising: a housing, the housing having disposed therein a workingelectrode, a reference electrode and a counter electrode, theelectrochemically active surface of the working electrode comprising agold film having a thickness in the range of approximately 1000 Å toapproximately 3000 Å and having been exposed to an atmosphere containinghydrogen sulfide of a sufficient concentration for a sufficient lengthof time to render it generally at least as sensitive to hydrogenchloride as to hydrogen sulfide, the electrochemical sensor furthercomprising an electrolyte present within the housing maintainingelectrical connection between the working electrode and the counterelectrode; and circuitry maintaining the working electrode at apotential in the range of approximately 1025 mV to approximately 1400 mVversus the normal hydrogen electrode.
 12. The electrochemical sensor ofclaim 11 wherein the circuitry maintains the working electrode at apotential in the range of approximately 1050 mV to approximately 1350 mVversus the normal hydrogen electrode.
 13. The electrochemical sensor ofclaim 11 wherein the circuitry maintains the working electrode at apotential in the range of approximately 1100 mV to approximately 1200 mVversus the normal hydrogen electrode.
 14. The electrochemical sensor ofclaim 11 wherein the thickness of the gold film is in the range ofapproximately 1900 Å to approximately 2800 Å.
 15. The electrochemicalsensor of claim 11 wherein the gold film is sputter coated over a porousmembrane.
 16. The electrochemical sensor of claim 11 wherein theelectrochemically active surface of the counter electrode comprisesplatinum.
 17. The electrochemical sensor of claim 11 wherein theelectrochemically active surface of the reference electrode comprisesplatinum.
 18. The electrochemical sensor of claim 11 wherein theelectrochemically active surfaces of both the reference electrode andthe counter electrode comprise platinum.
 19. The electrochemical sensorof claim 11 wherein the electrolyte comprises sulfuric acid.